One of the major challenges in studying development and aging is to elucidate the nature of the "developmental program" through which cells and organisms pass between birth and death. Insect metamorphosis involves a developmental program with many advantages for analysis. We are investigating the hormonal control of insect metamorphosis by following cuticle formation and circular protein synthesis in vitro in both short-term and long-term cultures. The roles of ecdysterone and juvenile hormone in controlling the switch between pupal and adult synthesis in imaginal discs which are committed to metamorphosis are being studied. Also under analysis is the relation between cellular proliferation and such differentiation. We are examinig the composition and enzymatic activity of individual cuticular proteins in order to define their contributions to structure, function, and development.